Document default log location in config files

It's not immediately obvious where log output will go if the 'log_file' and
'use_syslog' directives are either missing or commented out.  This patch updates
the documentation in the config files to mention the stdout default.  This
should help remove any confusion when configuring logging for glance.

Change-Id: I36f2a9d30bdee1949e60f413b7f181631c564ed5
Closes-Bug: #1248292
Merge "Document default log location in config files"